Radars with a low probability of interception or LPI—Low Probability of Intercept—are becoming more and more developed. It has been shown that these radars, although difficult to detect using conventional interceptors, can be detected by intel measurement systems specially dedicated to this end, for example by using an antenna having high rotational gain. There is therefore a need to strengthen the discretion of these so-called LPI radars so that they are as difficult as possible to detect.
Radar systems with a low probability of interception implementing a passive and active detection function using a rotating radar are known. Unfortunately, even if it is not possible to detect the passive interception function in principle, the system continues to be vulnerable if the radar can be detected when it is transmitting. Moreover, the rotating antennas make it necessary to use microwave rotary joints, which are generally critical components. Another disadvantage of rotating antennas is related to the noise generated by the rotation mechanisms of the antenna, which compromises the acoustic discretion of the radar. The transmission of very wideband digital and/or optical signals between the mobile part and the fixed part of the radar is likewise a problem.
Furthermore, when the environment does not allow the use of servomechanisms, such as when the system is intended to be mounted on a vehicle where there is a high risk of shocks, or when it is necessary to detect in a very short time, a system having a rotary directional antenna cannot be used, in view of the time required to sweep the whole of the angular field, typically a few seconds for 360°.
In general, LPI radars transmit pulses having low peak power in relation to classical pulse radars. Rather than transmitting pulses of short duration and high power, LPI radars have the special feature of distributing the power over time. They thus transmit pulses having low peak power but over a much longer time, and in the extreme case the transmission is continuous or quasi-continuous. A disadvantage of this transmission technique is that when it is used on its own it can be countered by detectors using, by way of example, processing that allows coherent integration of the signal over a long time, typically in the order of between one and a few tens of milliseconds.
Moreover, continuous wave radars have a major disadvantage, relating to the coupling between transmission and reception that are simultaneous, which tends to greatly limit the use of said radars when the range required is great, typically beyond between one and a few tens of kilometers.